1. Field of the Invention
The present invention relates to a focus detection apparatus for a camera or the like, having an auxiliary light source for illuminating an object to be photographed, for performing focus detection of plural object areas within a frame of a photographic lens.
2. Related Background Art
In a conventional focus detection apparatus for a camera, the following method of detecting a defocus amount of an object is known well. Light beams emitted from an object and passing through different exit pupil areas of a photographic lens are focused on a pair of line sensors, and a relative positional displacement between a pair of image signals obtained by photoelectrically converting an object image is obtained, thereby detecting the defocus amount of the object.
In the above method, since a pair of focus detection systems (optical systems and sensors) are used, only a defocus amount in one object area within the frame is detected. However, various methods are proposed wherein a plurality of detection systems are prepared to detect defocus amounts of plural object areas within the frame.
Even if plural object areas are subjected to focus detection, the number of object areas to be finally focused in a camera is one or two (in the later case, the photographic lens is operated to an intermediate focus position which covers, e.g., two areas in terms of the depth of field), an object area is selected in accordance with a given technique, and focusing of the photographic lens is performed by the defocus amount of the selected area.
Area selection is generally performed by a method in which an object area nearest to the camera is selected.
In this conventional focus detection apparatus, a defocus amount cannot be detected when the object has a low brightness level or when there is no change in brightness (reflectance) on a surface of an object even if the object brightness level is not low. For this reason, an auxiliary illumination unit is often used to illuminate the object in a focus undetectable state.
When plural focus detection object areas are present, it is convenient to arrange the illumination unit to illuminate these plural object areas.
In the conventional arrangement, however, the following problems are posed when focus detection of the plural object areas is performed using an illumination unit.
If a condition for operating the illumination unit is given such that all object areas are focus undetectable, the illumination unit is not operated even if one object area is focus detectable. For example, in a bright background with a person as an object within a frame, like a night scene, the illumination unit is not operated even if the object area in which the person is located is focus undetectable. In this case, the background is focused, resulting in inconvenience.
If a condition for operating the illumination unit is given such that at least one object area in a focus undetectable state is present, an object area located in the background is unnecessarily illuminated in a focus undetectable state, thus resulting in wasteful illumination.
In addition, if the illumination unit generates a common illumination pattern to all the object areas in favor of simplicity, it is difficult for some focus detection systems to detect such an illumination pattern in some object areas in principle. If such an area becomes detectable with an illumination, it often includes a large error in defocus amount because this object is not suitable for focus detection. When the defocus amount of this area is selected as a final defocus amount, a focusing error occurs.
In a conventional apparatus, since a focus undetectable state (low contrast) and a low brightness level are conditions for projecting auxiliary light, for example, even if a focus undetectable state occurs by an excessively large relative displacement of an image by a large defocus amount, auxiliary light is projected at a relatively low object brightness level. However, a focus detectable state cannot be obtained even if a focus undetectable state is caused by an excessively large defocus amount. A wasteful response time caused by an accumulation time in the presence of auxiliary light and wasteful energy consumption caused by an illumination occur.